The present invention relates generally to the processing of Fischer-Tropsch products and more particularly to a method for converting olefins and alcohols present in Fischer-Tropsch naphthas into ethers and blending the ethers into mid-distillate fuel components thereby improving the properties of the fuels.
The increased demand for middle distillate transportation fuels such as jet fuel and diesel fuel has provided the incentive for expanding the production of these fuels by converting natural gas, coal and heavy petroleum fractions. A known technique for processing these resources into distillate fuels involves a Fischer-Tropsch reaction whereby a synthesis gas essentially containing H2 and CO is converted into highly linear hydrocarbonaceous products containing paraffins, olefins and oxygenates such as acids and alcohols. The linear paraffins are converted into isoparaffinic distillate fuel components using known procedures such as hydrotreating, hydrocracking and hydroisomerization dewaxing. The isoparaffinic distillate fuels have excellent burning properties (high jet smoke points and high diesel cetane numbers) but are essentially free of oxygenates, aromatics and compounds containing hetero atoms and as a result, suffer from low lubricity, poor seal swell and low density.
The Fischer-Tropsch synthesis of hydrocarbonaceous products also provides a significant quantity of by-product naphthas. The naphthas are composed of low molecular weight linear hydrocarbons which are too volatile for incorporation into distillate transportation fuels. This naphtha stream is less valuable than distillate fuels but it is not possible to vary the reaction conditions to selectively eliminate the production of naphtha and increase the production of distillate transportation fuels. Furthermore, the naphtha often contains high levels of olefins and oxygenates which makes it unsuitable for use in gasoline or as a petrochemical plant feed. Accordingly, in conventional practice, the naphtha is refined to reduce the content of olefins and oxygenates in order to provide a salable naphtha.
Various solutions have been proposed to improve the lubricity of distillate fuels. The use of alcohols derived from a Fischer-Tropsch process in diesel fuels to control lubricity is described in U.S. Pat. No. 5,814,109. The production of mid-distillate fuels that contain oxygen (not oxygenates) with a high cetane number and good lubricity is described in European Patent Application EP 885275A1. U.S. Pat. No. 5,689,031 discloses the production of clean distillate fuel with improved lubricity by processing a Fischer-Tropsch wax. U.S. Pat. No. 6,087,544 discloses a process for producing distillate fuels having high lubricity and low sulfur levels by fractionating a distillate feedstream into a light fraction of relatively low lubricity which contains about 50 to 100 wppm of sulfur and a heavy fraction having a relatively high lubricity, hydrotreating the light fraction to remove substantially all of the sulfur and blending with the heavy fraction. U.S. Pat. No. 5,766,274 discloses producing a clean distillate useful as a jet fuel or jet blending stock with improved lubricity by separating a Fischer-Tropsch wax into heavier and lighter fractions, hydroisomerizing the heavier fraction and that portion of the light fraction boiling above about 475xc2x0 F., and blending. The isomerized product with the untreated portion of the lighter fraction. FR-0016538 describes the use of glycerol monoesters as lubricity improvers. SAE Paper 1999-01-1512 describes the use of conventional additives to improve the lubricity of a Fischer-Tropsch diesel fuel. The diesel fuel described in this report was prepared by a high temperature process followed by oligomerization. Diesel fuels made by this route will contain some aromatics and highly branched isoparaffins.
U.S. Pat. No. 4,547,601 describes the separation of water soluble oxygenates of a Fischer-Tropsch synthesis product from water and acids and subsequent conversion by a dehydration catalyst and a special zeolite catalyst to a middle distillate. See also related U.S. Pat. No. 4,260,841. U.S. Pat. No. 4,544,792 describes a process for converting olefinic feedstock, such as a Synthol olefinic liquid product of a Fischer-Tropsch synthesis, to distillate hydrocarbons by contacting the feedstock at elevated temperature and pressure with an acid zeolite conversion catalyst to oligomerize olefins and convert oxygenated hydrocarbons contained in said light oil. Temperatures up to 325xc2x0 C. and H2 are used thereby providing an effluent containing heavy distillate range hydrocarbon, light gas and byproduct water. Alcohols and oxygenates in the feed are removed prior to oligomerization by water washing. U.S. Pat. No. 4,398,050 describes the formation of methanol and higher alcohols with dehydration of the higher alcohols to make ethylene and propylene.
European Patent Application EP 1027409A1 describes the addition of ethers to compression ignition fuels. These ethers are described as being preferably those used in gasoline (which would include ethers with a high content of tertiary alkyl groups), di-ethyl ether, or those having less than 10 carbon atoms.
In a recent publication titled xe2x80x9cFrom Natural Gas to Oxygenates for Cleaner Diesel Fuels,xe2x80x9d presented at the 6th Natural Gas Conversion Symposium, Girdwood, Alaskaxe2x80x94Jun. 17-22, 2001, researchers from Snamprogetti S.p.A.xe2x80x94Milan, Italy described the preparation and use of di-n-pentyl and methyl-octyl ethers for use as a diesel fuel. The di-n-pentyl ether is prepared from butenes by a complex chemical process involving hydroformylation, and the methyl-octyl ether is prepared by a complex chemical process involving telomerization and selective hydrogenation.
U.S. Pat. No. 5,520,710 describes the use of certain symmetrical or unsymmetrical dialkyl ethers, dicycloalkyl ethers, or alkyl-cycloalkyl (polycycloalkyl) ethers containing a total of 2 to 24 carbon atoms, in combination with alkyl or dialkyl peroxides having one to 12 carbon atoms in each alkyl group, as supplements to diesel fuels to provide a cleaner burning fuel with significantly decreased hydrocarbon, carbon monoxide and particulate matter emissions. The supplements also significantly enhance the cetane number of the fuel and impart other desirable properties to the fuel, such as lowered pour and cloud points.
WO 99/21943 describes the blending of ethers with Fischer Tropsch diesel fuel. However, these ethers are described as having a carbon number of less than 10, commonly used in gasoline (methyl tertiary amyl ether or methyl tertiary butyl ether), and diethyl ether.
WO 01/46347 A1 discloses significantly improved reduced particulate emission performance of exhausts of vehicles powered by fuel combustion both at high and low loads by adding oxygenates or other hydrocarbon components in a diesel fuel composition comprising a major amount of a base fuel and a relatively minor amount of at least one chemical component other than that generated in a refinery process stream.
WO 01/46348 discloses a fuel composition comprising a base fuel having 50 ppm or less or sulfur, 10% or less of olefin, 10% or less of ester and at least 1 wt. % of oxygenate chosen from certain alcohols(s) and ketone(s) and having no other oxygen atom in its structure, with improved reduction of particulate emission without using further additives such as cyclohexane or peroxides or aromatic alcohol and with little to no increase in nitrogen oxide (NOx) emission at high engine loads.
It is an object of the invention to provide an improved process to prepare Fischer-Tropsch middle distillate fuels, specifically jet fuels and diesel fuels, having acceptable lubricity, seal swell and density.
It is another object of the invention to provide a procedure to increase the production of middle distillate Fischer-Tropsch fuels through processing of light Fischer-Tropsch naphthas.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, the claims appended thereto, and the Figures of the drawings.
The invention is based on the discovery that the production of highly isoparaffinic mid-distillate transportation fuels can be increased by treating a light Fischer-Tropsch naphtha to convert the olefins and alcohols present therein into dialkyl ethers, adding about 1 to 25% by weight of the ethers to highly paraffinic mid-distillate fuel fractions (i.e., those containing at least 70% by weight isoparaffins) obtained by a Fischer-Tropsch synthesis to improve lubricity and other properties, and recovering the olefin- and alcohol-reduced naphtha.
Distillate transportation fuels (diesel and jet fuels) containing dialkyl ethers obtained according to the invention have several desired physical properties: improved lubricity, improved seal swelling, good cetane numbers and good smoke points and good environmental properties (low water solubility and rapid biodegradability). By synthesizing ethers from Fischer-Tropsch lighter naphtha streams and blending them with Fischer-Tropsch transportation fuel components, the process of the invention increases the yield of desired distillate transportation fuel, and decreases the amount of refining that must be done to convert the naphtha into a salable product. Furthermore, the distillate fuels of this invention can be used as a distillate fuel blend component and blended with other distillate fuels components to form a salable distillate fuel. A salable distillate fuel is a jet or diesel fuel that meets all the applicable specifications for sale of that product in the country of sale. A distillate fuel blend component (either the product of this invention or other blend streams) does not necessarily need to meet all specifications, as when the blend is made, deficiencies in one can be compensated by properties of the other.